This invention pertains to the art of manufacturing TV funnels from molten glass, and more particularly to method and apparatus for inhibiting the formation of cracks or checks in the moil or yoke area of such funnels during the manufacturing process. In the hot forming of TV funnels from molten glass, the yoke or moil area adjacent the narrow relatively thin-walled neck portion of the funnel has a tendency to crack or check, during formation due to high compressive forces exerted by a plunger on relatively cool and viscous glass in such area, and during the withdrawal of the plunger due to mechanical abrading in the yoke area occasioned by plunger shifting during its withdrawal.
In the past it has been common practice to either press-form or spin-form TV funnels in glass molds having substantially uniform wall thicknesses along their entire extent. In addition, when press-forming such funnels, the plunger, although hollow for cooling purposes, normally had substantially uniform wall thicknesses comprising the entire plunger body.
U.S. Pat. No. 3,536,466 is representative of the prior art mold and plunger apparatus for forming funnel shaped articles, wherein the mold and plunger both have relatively thick or heavy wall portions of uniform wall thickness. Further, the patent illustrates the relatively large mass of metal concentrated about the yoke area of the funnel, which due to the fact that the glass is relatively thin in such area and due to the relatively high thermal conductivity inherent in metals, drastically cools the thin-walled glass in the yoke or moil area. In a like manner, U.S. Pat. No. 3,666,434 illustrates a conventional mold for spin-forming funnel shapes wherein the mold, although provided with cooling fins has substantially uniform relatively heavy wall portions along its extent. In addition, a relatively heavy knockout pin is shown which contributes to the loss of glass temperature in the moil area due to cooling by thermal conduction through the relatively heavy mold mass in such area.
When utilizing the uniform relatively thick-walled molding equipment of the prior art, the mass and high thermal conductivity of such walls absorbed much of the heat from the gob of molten glass delivered to the lower central or yoke area of the mold, particularly in view of the long residence time in which the glass is in contact with the yoke area. Due to the fact that the gob loses heat rapidly to the mold and a skin of cooled glass is formed adjacent the yoke area, larger pressing forces are required to flow the glass into conformity with the mold and form the desired article. Further, as the glass becomes chilled due to the extraction of heat therefrom, pressing forces cause the rather viscous and brittle glass to crack, particularly in the cooled moil area, which cracks deleteriously affect the strength and useability of the finished product. Further, as pressing forces increase, the tendency for the plunger to shift laterally is increased, which sidewise movement has a tendency to cause checks or cracks upon withdrawal of the plunger.
After forming the article it is necessary to cool the mold, and plunger when press forming, prior to removal of the finished article, and the relatively uniform thick-walled molding apparatus of the prior art requires substantial cooling time compared with the plunger and mold of the present invention. Further, the thick walls of the nose portion of the plunger prevented any useful contraction thereof during cooling for facilitating plunger withdrawal, whereas the relatively heavy yoke portion of the mold with a cooled exterior surface prevented useful expansion of such mold portion away from the glass for facilitating ware removal.